1160 
Journal of Agricultural Research 
Yol. XXVIII, No. 11 
The medium used was 
the following composition: 
Lipman and Brown's modified synthetic agar, havin 
Grams 
Dextrose_ 
MgS0 4 _ 
K 2 HP0 4 _ 
Peptone_ 
Agar_ 
FeS0 4 _ 
Distilled H 2 0 
10 . 0 
. 2 
. 5 
. 05 
15. 0 
Trace. 
1, 000 cc. 
The quantity of di-potassium phosphate used was just sufficient to make the 
reaction optimum for bacterial growth and hence no adjustment was necessary. 
After an incubation period of seven days at room temperature, the number of 
colonies appearing on each plate was determined. It happened that the dilu¬ 
tion used was somewhat low for the most accurate results, and furthermore, 
several plates were overrun by quick-growing fungi. The large number of plates 
poured for each treatment, namely 10 (duplicate beakers of soil and 5 plates 
from each beaker) largely overcame these sources of error, making the final 
results as accurate as may ordinarily be expected from quantitative bacteriological 
work. The average results for the various treatments, expressed in terms of 
millions of microorganisms per gram of moist soil, are given in Table I. 
The second set of samples for biological analysis was taken from the beakers 
of soil 31 days after the beginning of the incubation period. Plates were poured 
as previously except that a higher dilution was used. Three plates for each 
dilution were poured, making a total of 12 for each soil treatment. The average 
figures are given in Table I, together with those from the first sampling. 
From the data as given, it will be observed that there was no constant relation¬ 
ship between bacterial numbers as shown by the methods used and the known 
effects of the various materials on plant growth. All of the materials used 
increased the numbers of soil organisms. Of these materials, the two which are 
known to be the best sources of nitrogen for plants under widely varying con¬ 
ditions, namely, ammonium sulphate and urea, produced only moderate stimu¬ 
lations. In this connection, it is necessary to bear in mind that the urea bacteria 
do not grow well, if at all, on media such as the one used which did not contain 
urea. These urea bacteria probably were increased, but this could not be dis¬ 
covered by the method used. Dicyanodiamid, which is unavailable as a fertil¬ 
izer and injurious to nitrification, produced a moderate increase in numbers. 
Guanylurea sulphate, another material of doubtful agricultural value, caused 
greater increases than either of the three materials just mentioned. Cyanamid 
proved to be remarkably effective in increasing the rate of multiplication of the 
soil organisms, this increase amounting to 57.8 per cent within 16 days and 
320. 2 per cent after 31 days. On the other hand, very large quantities (2 per 
cent) practically sterilized the soil or at least so weakened the organisms that 
they were unable to develop on artifical media. 
Table I.— Numbers of microorganisms in soil following applications of cyanamid 
and related compounds 
Treatment 
No nitrogen_____ 
Ammonium sulphate 0.1 per cent ______ 
Urea (NO0.1 per cent ammonium sulphate) ... 
Dicyanodiamid (NO 0.1 per cent ammonium sulphate). 
Guanylurea sulphate (NO 0.1 per cent ammonium sulphate) 
Cyanamid (NO0.1 per cent ammonium sulphate). 
Cyanamid (NOO.5 per cent ammonium sulphate)_ 
Cyanamid (N02 per cent ammonium sulphate).. 
Millions per gram 
16 days 
31 days 
! 22.3 
22.8 
! 22.5 
31.7 
30.9 
27.2 
29.2 
25. 6 
1 28.6 
51.6 
35.2 
95.8 
42.7 
67.2 
. 05 
. 12 
